Gradually self-expanding stent

ABSTRACT

A gradually self-expanding stent is provided with a first region of stent cells. A restraining material is attached to the first region and restrains the first region from expanding to the expanded configuration. Over time the restraining material releases the first region to allow the first region to expand to the expanded diameter. The first region is located longitudinally between two second regions. The second regions expand at a faster rate than the first region.

This application is a continuation of U.S. patent application Ser. No.12/342,302, filed Dec. 23, 2008, which is incorporated herein byreference in its entirety.

BACKGROUND

The present invention relates generally to medical devices and moreparticularly to self-expanding stents.

Stents have become a common alternative for treating vascular conditionsbecause stenting procedures are considerably less invasive than otheralternatives. As an example, stenoses in the coronary arteries havetraditionally been treated with bypass surgery. In general, bypasssurgery involves splitting the chest bone to open the chest cavity andgrafting a replacement vessel onto the heart to bypass the stenosedartery. However, coronary bypass surgery is a very invasive procedurethat presents increased risk and requires a long recovery time for thepatient. By contrast, stenting procedures are performed transluminallyand do not require open surgery. Thus, recovery time is reduced and therisks of surgery are minimized.

Many different types of stents and stenting procedures are possible. Ingeneral, however, stents are typically designed as tubular supportstructures that may be inserted percutaneously and transluminallythrough a body passageway. Typically, stents are adapted to becompressed and expanded between a smaller and larger diameter. However,other types of stents are designed to have a fixed diameter and are notgenerally compressible. Although stents may be made from many types ofmaterials, including non-metallic materials and natural tissues, commonexamples of metallic materials that may be used to make stents includestainless steel and nitinol. Other materials may also be used, such ascobalt-chrome alloys, amorphous metals, tantalum, platinum, gold,titanium, polymers and/or compatible tissues. Typically, stents areimplanted within an artery or other passageway by positioning the stentwithin the lumen to be treated and then expanding the stent from acompressed diameter to an expanded diameter. The ability of the stent toexpand from a compressed diameter makes it possible to thread the stentthrough narrow, tortuous passageways to the area to be treated while thestent is in a relatively small, compressed diameter. Once the stent hasbeen positioned and expanded at the area to be treated, the tubularsupport structure of the stent contacts and radially supports the innerwall of the passageway. The implanted stent may be used to mechanicallyprevent the passageway from closing in order to keep the passageway opento facilitate fluid flow through the passageway.

One common type of stent used in medical procedures is theself-expanding stent. Self-expanding stents are usually made of shapememory materials or other elastic materials that act like a spring.Self-expanding stents are increasingly being used by physicians becauseof their adaptability to a variety of different conditions andprocedures. Typical metals used in this type of stent include nitinoland stainless steel. However, other materials may also be used. Tofacilitate stent implantation, self-expanding stents are normallyinstalled on the end of a delivery catheter in a low profile, compressedstate. The stent is typically inserted into a sheath at the end of thecatheter, which restrains the stent in the compressed state. The stentand catheter assembly is then guided along a guide wire to the portionof the vessel to be treated using the Seldinger technique, which is wellknown in the art. Once the catheter and stent are positioned adjacentthe portion of the vessel to be treated, the stent is released bypulling, or withdrawing, the sheath rearward. Normally, a stop or otherfeature is provided on the catheter to prevent the stent from movingrearward with the sheath. After the stent is released from the retainingsheath, the stent springs radially outward to an expanded diameter untilthe stent contacts and presses against the vessel wall. Generally,self-expanding stents are selected such that the expanded outer diameterof the stent is greater than the inner diameter of the blood vessel. Inthis way, the continuous outward force of the stent against the innersurface of the blood vessel helps to hold the stent in the deploymentlocation and prevent migration of the stent through the vessel.

Traditionally, self-expanding stents have been used in a number ofperipheral arteries in the vascular system due to the elasticcharacteristic of these stents. However, they may be used in thecoronary, carotid, femoral, and renal arteries as well. One advantage ofself-expanding stents for peripheral arteries is that stresses fromexternal sources do not permanently deform the stent. As a result, thestent may temporarily deform during unusually harsh stresses and springback to its expanded state once the stress is relieved. However,self-expanding stents may be used in many other currently known or laterdeveloped applications as well.

Self-expanding stents are commonly used in angioplasty, or themechanical widening of narrowed or completely obstructed blood vessels.Typically, the blood vessels are narrowed or obstructed as a result ofarteriolosclerosis or atherosclerosis. Angioplasty is generallyperformed using a balloon that is tightly folded around a catheter. Thecatheter is delivered to the treatment site using the aforementionedSeldinger technique, and the balloon is inflated with a fluid, typicallysaline, contrast, or a mixture thereof. The fluid is injected into theballoon using pressures that are much higher than normal blood pressuresuntil the balloon is inflated to a fixed, predetermined size. This highpressure inflation of the balloon forces the vessel wall at thetreatment site to expand in a radially outward direction, therebywidening the obstructed portion of the blood vessel.

Once the blood vessel has been expanded, a self-expanding stent isdelivered to the treatment site and deployed into the blood vessel inthe above described manner. Because the self-expanding stent acts like aspring, once the stent is released from the delivery catheter itimmediately expands to the inner diameter of the blood vessel andcontinuously exerts outward pressure against the vessel wall. While thisoutward pressure helps to maintain the position of the stent, the suddenapplication of outward pressure by the stent against the wall of avessel that has just undergone balloon expansion may further traumatizethe vessel tissue. Such trauma may give rise to potential problems suchas hyperplasia, or the abnormal proliferation of cells at the treatmentsite.

Current research has shown that hyperplastic response to angioplasty orstenting appears to be greatly increased in vessels where the internalelastic lamina (“IEL”) layer of the vessel is ruptured during theangioplasty or stenting procedure. In areas where the IEL is ruptured ordamaged, the blood vessel usually exhibits a healing inflammatoryresponse in the form of neointimal growth (abnormal increased growth ofcells), which may lead to restenosis, or re-narrowing of the bloodvessel. In contrast, areas of the blood vessel where the IEL is leftintact tend not to exhibit such neointimal growth. Consequently, it ispreferable to leave the IEL intact during the stenting procedure inorder to reduce hyperplasia/neointimal growth.

One attempt to reduce stent caused trauma to blood vessel tissue isillustrated by U.S. Pat. No. 6,613,077 to Gilligan et al. Gilligan etal. proposes the use of biodegradable sutures circumferentially woundtightly around the exterior of a stent. After the stent is deployed in avessel, the constraining sutures restrain the stent from fullyexpanding. As the sutures begin to biodegrade they yield and then break,thereby releasing the stent against the vessel wall. However, becausethe sutures are wrapped around the circumference of the stent, once thesuture fails, the entire circumference of the stent immediately andsuddenly expands to contact the vessel wall.

Similarly, U.S. Pat. No. 7,022,132 to Kocur proposes biodegradable bandsthat are wrapped around the exterior of the stent or interwoven aroundthe circumference of the stent. Kocur also proposes biodegradable bandswrapped around two individual stent struts. In each case, the bands aredesigned to initially hold the stent in a compressed shape. As the bandsbiodegrade they fracture, thereby immediately and suddenly releasing thestent against the vessel wall.

However, these techniques may unnecessarily traumatize the IEL bysuddenly expanding against the vessel wall when the sutures or bandsfail. Further, these techniques present significant manufacturingobstacles as the sutures and bands must be wound, woven, and/or tiedaround the circumference of the stent or individual stent struts to keepthe stent or portions thereof in a compressed configuration. It hasbecome apparent to the inventor that an improved stent would bedesirable.

SUMMARY

Self-expanding stents are described which may allow for more gentle andgradual expansion against a vessel wall. The gradually self-expandingstent includes a restraining material that covers and restrains at leastsome stent cells in a compressed configuration after deployment in abody lumen. Additional details and advantages are described below in thedetailed description.

The invention may include any of the following aspects in variouscombinations and may also include any other aspect described below inthe written description or in the attached drawings.

In one aspect, a gradually self-expanding stent includes a stentstructure formed from a series of structural members that are designedto flex between expanded and compressed configurations. The stentstructure has a generally cylindrical shape in its uncompressedconfiguration. The self-expanding stent also includes a first pluralityof stent cells, each stent cell having an area defined bycircumferentially adjacent structural members that are in mechanicalcommunication with one another. When the circumferentially adjacentstructural members of the stent cells are in the compressedconfiguration, the stent cells have a first area. When the structuralmembers of the stent cells are in the expanded configuration, the stentcells have a second area that is larger than the first area.

A restraining material substantially covers the area of a secondplurality of stent cells. The restraining material preferably restrainseach of the plurality of stent cells in the compressed configuration.The restraining material may be attached to the circumferentiallyadjacent structural members defining each stent cell in the secondplurality of stent cells. Over time, the restraining material isconfigured to release the structural members defining the stent cells,thereby allowing the stent cell to assume its expanded configuration.

In one aspect, the restraining material may be biodegradable. In anotheraspect, the restraining material may be stretchable. In yet anotheraspect, the restraining material may not extend around the circumferenceof the stent structure in a continuous manner.

In another embodiment, the gradually self-expanding stent may include astent structure having a proximal end portion, a distal end portion, anda central portion. The proximal end portion may extend toward alongitudinal center of the stent structure from a proximal end thereofby an amount based on an overall length of the stent structure, whilethe distal end portion may extend toward the longitudinal center of thestent structure from a distal end thereof by an amount based on anoverall length of the stent structure. The central portion is disposedbetween the proximal and distal end portions. Preferably, the stentcells of the proximal and distal end portions are comprised of the firstplurality of stent cells, while the stent cells of the central portionare comprised of both the first and second plurality of stent cells.

In one aspect, a proportion of the first plurality of stent cells to thesecond plurality of stent cells in the central portion is substantiallyequal. The central portion may also include an intermediate portion,wherein the restraining material covering stent cells disposed in theintermediate portion is configured to release the structural membersmore quickly than the restraining material for stent cells disposedoutside of the intermediate portion. The intermediate portion mayinclude a proximal intermediate portion and a distal intermediateportion. The proximal intermediate portion preferably extends toward thelongitudinal center of the stent structure from a distal end of theproximal end portion by a predetermined amount, while the distalintermediate portion extends toward the longitudinal center of the stentstructure from a proximal end of the distal end portion by thepredetermined amount. The predetermined amount is preferably based on alength of the central portion.

In another aspect, the restraining material of stent cells disposed inthe intermediate portion may be configured to release the structuralmembers more quickly than the restraining material of stent cellsdisposed outside of the intermediate portion.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The presently preferred embodiments, together with furtheradvantages, will be best understood by reference to the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more fully understood by reading the followingdescription in conjunction with the drawings, in which:

FIG. 1 is a close-up side view of a plurality of stent cells of aself-expanding stent in a compressed state;

FIG. 2 is a close-up side view of a plurality of stent cells of analternative embodiment of the self-expanding stent of FIG. 1 in acompressed state;

FIG. 2( a) is a close-up side view of a plurality of stent cells of theembodiment of FIG. 2 showing an alternative configuration of arestraining material;

FIGS. 3( a)-(f) are side views of various embodiments of theself-expanding stent of FIG. 1 in a compressed state;

FIG. 4 is a side view of an embodiment of a self-expanding stent in acompressed state before deployment in a body lumen;

FIG. 5 is a side view of the self-expanding stent of FIG. 4 includingbiodegradable restraining material in an initial expansion stage afterdeployment within a body lumen;

FIG. 6 is a side view of the self-expanding stent of FIG. 5 in anintermediate expansion stage after deployment;

FIG. 7 is a side view of the self-expanding stent of FIG. 5 in a finalexpansion stage after deployment;

FIG. 8 is a side view of the self-expanding stent of FIG. 4 including astretchable restraining material in an initial expansion stage afterdeployment within a body lumen;

FIG. 9 is a side view of the self-expanding stent of FIG. 8 in anintermediate expansion stage after deployment;

FIG. 10 is a side view of the self-expanding stent of FIG. 8 in a finalexpansion stage after deployment;

FIG. 11 is a side view of another embodiment of a graduallyself-expanding stent;

FIG. 12 is a side view of an another alternative embodiment of agradually self-expanding stent;

FIG. 13 is a partial side cross-sectional view of the graduallyself-expanding stent of FIG. 12 in an initial expansion stage afterdeployment within a body lumen.

DETAILED DESCRIPTION

Referring now to the figures, FIGS. 1-3 illustrate a graduallyself-expanding stent according to an embodiment of the presentinvention. The gradually self-expanding stent structure 100, 200 isparticularly useful in lesions or areas where the narrowed or completelyobstructed blood vessels are chronic rather than acute such that agradual opening of the vessel would be clinically acceptable, forexample and without limitation, peripheral arteries. Various designsknown in the art may be used for the stent structure 100, 200. Forexample, the stent structure 100, 200 may be made with serpentine ringsinterconnected with longitudinal structural members. The stent structure100, 200 may be fabricated from a cannula, and may have longitudinalsegments of laterally interconnected closed cells, as disclosed in U.S.Pat. Nos. 6,231,598, and 6,743,252 which are assigned to Cook Inc., theassignee of the present invention and are hereby incorporated byreference in their entirety. Each of the closed cells may be at leastpartially defined laterally by a pair of longitudinal struts that areinterconnected at each end by a circumferentially adjustable member.Each of the pair of longitudinal struts deforms or flexes to permitcircumferential expansion while the length of the cell is maintained bythe longitudinal struts. The stent may include adjacent longitudinalsegments that are joined by flexible interconnection segments therebypermitting the stent to bend laterally and that are comprised ofcurvilinear struts that form a series of serpentine bends. Thecurvilinear struts may distribute lateral bending forces while onlyallowing a slight overall shortening of the stent structure 100, 200.

Alternatively, the stent structure 100, 200 may be a wire frameconstructed from a plurality of wire stent segments as disclosed in U.S.Pat. No. 5,195,984. The stent structure 100, 200 may also be made from abraided framework of wire filaments. Other well-known stent structuresare also possible. Various materials may be used for the self-expandingstent structure 100, such as nitinol or stainless steel.

FIG. 1 illustrates embodiments of the stent structure 100. The stentstructure 100 is comprised of a plurality of interconnected structuralmembers 110 that define a plurality of stent cells 130. Specifically,each stent cell 130 is defined by circumferentially adjacent structuralmembers 120. As shown in FIG. 1, a pair of upper and lower structuralmembers 120 may be connected to each other through apexes ortransitional portions, or may be directly connected to each other. Eachstent cell 130 has an area that is defined by the individual structuralmembers 120 of each stent cell 130. Thus, in the embodiment shown inFIG. 1, each stent cell 130 has a shape and area resembling aparallelogram. However, it should be understood that the shape andcorresponding area of the stent cells 130 are not limited thereto sincethe stent cell's 130 shape and area will substantially correspond to thegeometric relationship between the circumferentially adjacent structuralmembers 120.

The structural members 120 of each stent cell 130 are configured to flexbetween a compressed configuration and an expanded configuration. Whenthe stent cells 130 are in the compressed configuration, the upper andlower pairs of circumferentially adjacent structural members 120 form anacute angle with one another such that an area of the stent cell 130 isminimized. In contrast, when the stent cells 130 are in the expandedconfiguration, the upper and lower pairs of circumferentially adjacentstructural members 120 form a less acute, or even a right or obtuseangle between one another such that the area of the stent cell 130 ismaximized. Note that the compressed configuration need not correspond toa maximally compressed configuration, and may refer to any intermediateconfiguration between the maximum compressed and expanded states,provided that the outer diameter of the stent structure 100 is smallerin the compressed configuration than in the expanded configuration.Similarly, the expanded configuration need not correspond to a maximallyexpanded configuration and may refer to any intermediate configurationbetween the maximum expanded and compressed states, provided that theouter diameter of the stent structure 100 is larger in the expandedconfiguration than in the compressed configuration.

Preferably, the structural members 110 of the stent structure 100 aremade from elastic, super-elastic, or spring-metal alloys such asnitinol, stainless steel, cobalt chromium, nickel titanium, platinum,inconel, or any other material known in the art, such that thestructural members 110 can compress under force, and when unrestrainedwill tend to return to the expanded configuration in a spring-likemanner. Note that adjacent stent cells 130 may share some of the samestructural members 120. That is, each of the structural members 110 maydefine two separate stent cells 130, one on each side of the structuralmember 110.

A restraining material 140 may be applied to some, but preferably notall of the stent cells 130 in the compressed configuration. Preferably,the restraining material is attached to each of the circumferentiallyadjacent structural members 120 and extends across the entirety of thearea of the stent cell 130, thereby connecting the circumferentiallyadjacent structural members 120 to one another and restraining thestructural members 120 in the compressed configuration. Because therestraining material 140 is attached to, and extends between each of thestructural members 120, the restraining material 140 creates arestraining structure that holds the circumferentially adjacentstructural members 120 of the stent cell 130 in the compressedconfiguration. Thus, the restraining material 140 prevents thestructural members 120 from assuming the expanded configuration, evenwhen the stent structure is no longer exposed to restraining orconstraining forces in a radially inward direction.

Turning to FIGS. 2 and 2( a), a stent structure 200 is comprised of aplurality of ring structures 260 connected by connecting members 250.Each of the stent rings 260 is comprised of a plurality of structuralmembers 210 that are connected by apexes or connecting portions suchthat the structural members 210 form an undulating or serpentinestructure. As with the stent structure 100 of FIG. 1, the structuralmembers 210 define a plurality of stent cells 230. Specifically, eachstent cell 230 is at least partially defined by circumferentiallyadjacent structural members 220. The circumferentially adjacentstructural members 220 may be directly connected to one another, or maybe connected via an apex or connecting portion such that thecircumferentially adjacent structural members 220 are in mechanicalcommunication with each other. In this configuration, each of the upperand lower circumferentially adjacent structural members 220 defines atleast a portion of the cell 230. A pair of upper and lower structuralmembers 220, which may be directly connected or connected through apexesor transitional portions, define at least a portion of the edge(s) of anarea of a stent cell 230.

As with the stent structure 100 of FIG. 1, the structural members 210are configured to flex between a compressed configuration and anexpanded configuration. Thus, when the stent structure 200 is in thecompressed configuration, the upper and lower pairs of circumferentiallyadjacent structural members 220 are disposed substantially adjacent toand abutting one another such that an area of the stent cell 130 isminimized. In contrast, when the circumferentially adjacent structuralmembers 220 are in the expanded configuration, the upper and lowerstructural members 220 are angled away from each another such that thearea of the stent cell 230 is maximized. As with the stent structure 100of FIG. 1, it should be understood that the term compressedconfiguration need not correspond to a completely compressedconfiguration where the circumferentially adjacent structural members220 abut each other. Thus the compressed configuration may refer to anyintermediate configuration between the maximally compressed and expandedstates, provided that the outer diameter of the stent structure 200 issmaller in the compressed configuration than in the expandedconfiguration. Similarly, the term expanded configuration need notcorrespond to a maximally expanded configuration and may refer to anyintermediate configuration between the maximally expanded and compressedstates, provided that the outer diameter of the stent structure 200 islarger in the expanded configuration than in the compressedconfiguration.

A restraining material 240 may be applied to some, but preferably notall of the stent cells 230 in the compressed configuration. Preferablythe restraining material is attached to at least each of thecircumferentially adjacent structural members 220 and extends at leastacross the space between the structural members 220. Because therestraining material 240 is attached to, and extends between each of thestructural members 220, the restraining material 240 creates arestraining structure that holds the circumferentially adjacentstructural members 220 of the stent cell 230 in the compressedconfiguration. Thus, the restraining material 240 prevents thestructural members 220 from assuming the expanded configuration, evenwhen the stent structure is no longer exposed to compressive forces.

As discussed above, the stent cell area is at least partially defined bythe circumferentially adjacent structural members 220. In the embodimentshown in FIG. 2, the area of the stent cell 230 is defined primarily bythe two circumferentially adjacent structural members 220, and therestraining material 240 extends therebetween. Preferably, therestraining material 240 is attached to the circumferentially adjacentstructural members 220 and extends across the entirety of the area ofthe stent cell 230, as shown in FIG. 2.

FIG. 2( a) illustrates another embodiment in which the area of the stentcell is defined by two pairs of circumferentially adjacent structuralmembers 220, each pair disposed on an adjacent ring structure 260. Thetwo pairs of circumferentially adjacent structural members 220 may havea mirrored configuration in the longitudinal direction, as shown in FIG.2( a), or may have the same longitudinal configuration. The restrainingmaterial is preferably attached to at least the structural members 220of the stent cell 230. The restraining material 240 may extend acrossthe entirety of the area of the stent cell 230, thereby connecting thecircumferentially adjacent structural members 220 of each ring structure260 to one another, as well as connecting the adjacent ring structures260. Accordingly, the restraining material 240 acts to restrain thestructural members 220 of the stent cells 230 from expanding afterdeployment in a body lumen, while simultaneously restraining relativemovement between the adjacent ring structures 260.

The restraining material 140, 240 may be rigid or may be stretchable(e.g. capable of stretching or yielding through elastic or plasticdeformation, or a combination thereof). Where the restraining materialis stretchable, the restraining material may be made of biodegradablematerials, nonbiodegradable materials, or a combination thereof. Forexample, a nonbiodegradable restraining material 140, 240 havingstretchable characteristics may include elastic impregnated ePTFE.

Preferably, the restraining material is made from a biodegradable orbioabsorbable material that is configured to degrade or dissolve overtime when exposed to body tissue and/or fluids. The restraining materialmay be made from one or more biodegradable polymers in varyingcombinations, such as polymers, copolymers, and block polymers. Someexamples of such biodegradable (i.e. bio-resorbable/bioabsorbable)polymers include poly-lactic acid, polyglycolic acid, polyglycolides,polylactides, co-polymers of polyglycolides and polylactides,polycaprolactones, polyglycerol sebacate, polycarbonates (e.g. tyrosinederived polyethylene oxide), polybutylene terepthalate, polydioxanones,hybrids, composites, collagen matrices with growth modulators,polyanhydrides, polyorthoesters, chitosan, aliginates, proteoglycans,glycosaminoglycans, vacuum formed SIS (small intestinal submucosa),fibers, chitin, and dextran. Any of these biodegradable polymers may beused alone or in combination with these or other biodegradable polymersin varying compositions. Examples of biodegradable materials exhibitingrigid characteristics include chitosan, alginates, and co-polymers ofpolyglycolides and polylactides. Examples of biodegradable materialsexhibiting stretchable characteristics include co-polymers ofpolycaprolactone with polyethylene, polypropylene, polyamids, orpolyester.

In the case of rigid biodegradable restraining materials 140, 240, asthe material degrades or dissolves, the radially outward force exertedby the structural members 110, 120 will eventually cause the restrainingmaterial 140, 240 to fracture when the force of the stent exceeds theultimate tensile strength of the restraining material 140, 240. In theevent the restraining material 140, 240 fractures such that one or moreof the circumferentially adjacent structural members 120 is no longerconnected to the other structural members 120, the disconnectedstructural member 120 is no longer restrained, and the energy storedwithin the structural member is released as the structural member 120assumes its expanded configuration. Once the restraining material 140,240 dissolves or fractures to the point that all of thecircumferentially adjacent structural members 120 become disconnectedfrom one another, the energy stored in each of the structural members120 is released, and the stent cell 130, 230 assumes its expandedconfiguration.

In the case where the restraining material 140, 240 is a stretchablematerial, the stent cells 130, 230 achieve the expanded configurationthrough plastic or elastic deformation rather than through failure ofthe restraining material 140, 240. As with the rigid restrainingmaterial 140, 240, the stretchable restraining material 140, 240 iscontinually subjected to radially outward expansive force from thecircumferentially adjacent structural members 120. The restrainingmaterial 140, 240 is configured to hold its initial shape during andshortly after deployment in a body lumen. However, over time, theoutward force exerted by the structural members 120 causes thestretchable material to elongate or stretch in a gradual and controlledmanner, thereby allowing the area of the attached stent cell 130, 230 toincrease. The stretchable restraining material 140, 240 may be anonbiodegradable material that is configured to stretch over time solelydue to the elastic properties of the material, or the restrainingmaterial may be a biodegradable material that is configured to stretchas the restraining material 140, 240 degrades. However, it should beunderstood that the stretchable restraining material 140, 240 maystretch over time due to both the elastic properties of the material andthe degradation of the material. Furthermore, it should be understoodthat the stent cells 130, 230 may expand through a combination ofstretching and fracturing of the restraining material 140, 240. In thiscase, the restraining material 140, 240 may expand through stretching inone expansion stage and through fracture during another expansion stage.

As the area of the stent cells 130, 230 increase in size due to eitherthe stretching or fracture of the restraining material 140, 240, thestent structure 100, 200 slowly expands from the compressedconfiguration, in which the portions of the stent structure 100, 200containing stent cells 130, 230 covered by the restraining material 140,240 do not abut the inner surface of a wall of the body lumen, to theexpanded configuration where exterior surface of the stent structure100, 200 abuts and engages the wall of the body lumen.

FIGS. 3( a)-(f) illustrate a plurality of exemplary patterns in whichthe restraining material 140, 240 may be applied to the stent cells 130,230. Typically, the expanded, unrestrained diameter of the stent isselected by physicians such that the stent will appose the wall of theconstricted vessel in its fully expanded form, while not migratingdownstream. The length of the stent structure 100, 200 is generallyspecified such that it will cover the entirety of a lesion ornarrowed/obstructed portion of the blood vessel and extend from healthytissue to healthy tissue disposed on opposite ends thereof.

The stent structure 100, 200 may be divided into three expansionsections: a proximal end portion 310 that extends from the proximal end340 of the stent structure 100, 200 to the proximal end of a centralportion; the central portion 330 that extends from the distal end of theproximal end portion 310 to the proximal end of the distal end portion320; and the distal end portion 320 that extends from the distal end ofthe central portion to the distal end 350 of the stent structure 100,200. In order to ensure the stent structure 100, 200 is firmly fixedagainst healthy tissue at one or both ends of the stent structure 100,200, it is preferable that the restraining material 140, 240 is notapplied to the stent cells 130, 230 of the proximal and distal ends 310,320 of the stent structure 100, 200. In this way, when the stentstructure is deployed in a body lumen, the proximal and distal ends 310,320 of the stent structure 100, 200 are free to fully and immediatelyexpand outward to appose the healthy tissue of the body lumen, therebyfixing the stent in place.

The proximal and distal end portions 310, 320 of the stent structure100, 200 preferably have sufficient length to allow the struts 120, 220to expand out and appose the healthy tissue of the body lumen (bloodvessel, etc.) with sufficient force to maintain the position of thestent structure 100, 200. For example, for stents having a length ofless than thirty (30) millimeters the proximal end portion 310 and thedistal end portion 320 may have a length of five (5) millimeters orless. Stent structures having a length of greater than thirty (30)millimeters (e.g. 50 mm, 100 mm, 140 mm, etc.) may have proximal anddistal end portions 310, 320 measuring less than or equal to ten (10)millimeters.

As shown in FIGS. 3( a)-(f), the majority of stent cells 130, 230covered by the restraining material 140, 240 are preferably located inthe central portion 330 of the stent structure 100, 200. In this way,when the stent structure 100, 200 is deployed in a blood vessel, theends of the stent structure are able to immediately expand out andappose the vessel wall, while the central portion 330 of the stentstructure 100, 200 remains substantially in the compressed configurationdue to the restraining material 140, 240. Preferably, the pattern ofrestrained stent cells 130, 230 does not extend in a continuous manneraround the entire circumference of the stent structure at any givenlongitudinal position along the stent structure 100, 200. Thus, therestraining material does not form a continuous circumferential band ofrestraining material at any given longitudinal position along the lengthof the stent structure 100, 200. In this way, when a single or a groupof stent cells 130, 230 expands through stretching or fracturing of therestraining material 140, 240, the expansion of the stent structure 100,200 is limited to that portion of the stent. Because the expansion ofthe stent structure 100, 200 is localized, the entire circumference ofthe stent structure 100, 200 is less likely to suddenly or immediatelyexpand and apply significant force to the tissue of thelesion/stricture, thereby minimizing the potential of rupturing the IEL.

Preferably, the restraining material 140, 240 is only applied to aplurality of stent cells 130, 230 disposed in the central portion 330 ofthe stent structure 100, 200. In this way, the proximal and distal endportions of the stent structure 100, 200, which contain only open cells130, 230, are allowed to expand out to the inner diameter of the bodylumen (e.g. blood vessel, etc.) upon deployment, while the centralportion 330 remains partially restrained such that the overall profileof the stent structure 100, 200 assumes an hourglass-like shape.

As shown in FIGS. 3( a)-(f), the restraining material 140, 240 ispreferably applied in a substantially uniform manner such that roughly50% of the total number of stent cells 130, 230 in the central portion330 are covered by the restraining material 140, 240. However, it shouldbe understood that the present invention is not limited thereto, and therestraining material 140, 240 may be applied to more or less than 50% ofthe stent cells 130, 230 of the central portion 330 in order to adjustthe degree of initial expansion of the stent structure 100, 200 upondeployment. In particular, embodiments having 25-30% of the stent cells130, 230 disposed in the central portion 330 restrained by therestraining material 140, 240 for less restrained configurations, andembodiments having 50-75% of the stent cells 130, 230 disposed in thecentral portion 330 restrained by the restraining material 140, 240 formore restrained configurations are contemplated.

Because some of the stent cells 130, 230 in the central portion 330 areunrestrained by the restraining material 140, 240, the unrestrainedcells 130, 230 are free to assume their fully expanded configurationwhen the stent structure 100, 200 is deployed (released) in a bodylumen. However, because some of the stent cells 130, 230 are restrainedin their compressed configuration by the restraining material 140, 240,the overall diameter of the central portion 330 of the stent structure100, 200 is initially only able to expand partially upon deployment.Preferably, the stent structure 100, 200 is only able to expand to 30%to 50% of its unrestrained diameter in an initial state afterdeployment. Preferably, the degree of initial expansion is selectedbased on the characteristics of the stricture/lesion the stent structure100, 200 is intended to treat. That is, the stent structure 100, 200 ispreferably selected/configured such that the stent structure 100, 200contacts, but does not apply significant outward force against the innersurface of the stricture/lesion in an initial state. Because the stentstructure 100, 200 initially only applies minimal force against the IELof the stricture/lesion, the IEL is less likely to rupture or beotherwise damaged.

For example, for a stricture occluding 70% of a blood vessel, anappropriate number of stent cells 130, 230 would be covered by therestraining material 140, 240 to allow the stent structure 100, 200 toinitially expand to 30% of the vessel diameter. Note that stentstructures 100, 200 are typically specified such that the maximum/fullyexpanded diameter is greater than the unrestricted diameter of the bodylumen the stent structure 100, 200 is intended to treat. Accordingly, inthis example, if the stent structure 100, 200 is configured to expand to30% of the diameter of the blood vessel, the stent structure 100, 200will be configured have an initial expansion of less than 30% of itsmaximum/fully expanded diameter, for example and without limitation,25%.

It is also preferable that the restraining material 140, 240 be appliedto stent cells 130 in a uniform pattern such that when the stentstructure 100, 200 is deployed in a body lumen, the stent structure 100,200 expands in a uniform manner in the radial and longitudinaldirections. Uniform radial and longitudinal expansion provides severaladvantages in both manufacture and operation of the stent structure 100,200. For example, if the stent structure 100, 200 has uniform expansioncharacteristics, the stent structure 100, 200 may be loaded into adelivery system (e.g. delivery catheter, etc.) from either end withoutaffecting performance. That is, in the preferred embodiment where bothends of the stent structure 100, 200 are functionally identical, theproximal end 340 and the distal end 350 of the stent are merelydetermined by the orientation of the stent structure 100, 200 in thedelivery system (i.e. the end of the stent that is disposed closest tothe proximal end of the delivery system is the proximal end 340 of thestent structure 100, 200 and the end closest to the distal end of thedelivery system is the distal end 350 of the stent structure 100, 200).Thus, complex orientation control systems to ensure that stentstructures 100, 200 are loaded into delivery systems from a particularend can be avoided.

Further, because the stent structure 100, 200 expands in a longitudinaland radially uniform manner, a physician does not have to select a stentstructure 100, 200 that matches a particular longitudinal profile of alesion/stricture; rather, the physician is able to specify a stentstructure 100, 200 by simply selecting a stent having an initialexpansion diameter, which is a function of the number of stent cells 130restrained by the restraining material 140, 240, that is just smallerthan, or preferably equal to the narrowest portion of thelesion/stricture. Moreover, the uniform initial expansioncharacteristics of the stent structure 100, 200 offer advantages inplacement of the stent as the physician need only place the stentstructure 100, 200 according the axial position of the lesion in thebody lumen and without regard for the particular profile of the lesion.However, it should be understood that the restraining material 140, 240may be applied to the stent structure 100, 200 in a non-uniform pattern,particularly in stents exceeding thirty (30) millimeters in length.

The restraining material 140, 240 of different stent cells 130, 230 maybe configured to degrade or stretch at different rates and times,depending on the thickness or chemical/physical makeup of the material.That is, the restraining material 140, 240 covering the area of somestent cells 130, 230 may be configured to degrade/stretch at a faster orslower rate as compared to the restraining material 140, 240 coveringother stent cells 130, 230. As shown in FIG. 11, a stent structure 1100,which includes a plurality of open and unrestrained cells 1136 and aplurality of restrained stent cells, may be divided into three types ofexpansion regions: end regions 1160, intermediate regions 1170, and acentral region 1180. The end regions 1160 correspond to the proximal anddistal portions of the stent structure 1100. The end regions 1160preferably extend inward a distance (a) toward the center of the stentstructure 1100 from the proximal and distal ends 1140, 1150,respectively. For example, the distance (a) may be less than or equal tofive (5) millimeters for stents having a length of thirty (30)millimeters or less, or ten (10) millimeters or less for stents having alength of (30) millimeters or more. The stent cells 1130 in the endregions 1160 are preferably open and unrestrained by a restrainingmaterial, such that they are free to immediately expand to theiruncompressed diameter when released.

The intermediate regions 1170 extend toward the longitudinal center ofthe stent structure 110 by a length (b) from the distal end of theproximal end portion 1160 and the proximal end of the distal end portion1160. Preferably, the length (b) is between 10-30% of the total length(d) between the proximal end of the distal end region 1160 and thedistal end of the proximal end region 1160. For example, for a stenthaving a length (d) of eighty (80) millimeters, the length (b) may bebetween eight (8) and twenty-four (24) millimeters in length.

The stent cells 1130 in the intermediate region 1170 are covered by arestraining material 1134 having a first degradation/stretching rate.For example, the restraining material 1134 may be configured todegrade/stretch such that the stent cells 1130 in the intermediateregion 1170 are completely released and free to assume the expandedconfiguration in, for example, 1-4 weeks, and preferably between 3-4weeks.

The central region 1180 corresponds to a region centered in thelongitudinal center of the stent structure 1100 and has a length (c)that extends between, and abuts the intermediate regions 1170.Preferably, the length (c) is between 40 to 80% of the total length (d)between the proximal end of the distal end region 1160 and the distalend of the proximal end region 1160.

The stent cells 1130 in the central region 1180 are covered by arestraining material 1132 having a second degradation rate that isslower than the first degradation rate of the restraining material 1134of the intermediate portion 1170. Preferably, the second degradationrate is a whole integer multiple of the first degradation rate, e.g. 2×,3×, etc. For example, if the restraining material 1134 is configured todegrade/stretch in 3 weeks, the restraining material 1132 may beconfigured to degrade/stretch in 6 weeks, or twice the amount of timethe restraining material 1134 takes to degrade/stretch. Thus, over time,due to their faster degradation/stretching rate, the intermediateportions 1170 provide a smooth transition portion between theunrestricted end portions 1160 and the slower degrading/stretchingcentral portion 1180. This configuration results in an hourglass typeshape that allows the stent structure 1100 to gradually exert outwardradial force against an inner surface of the lesion/stricture, whichhelps to reduce trauma to the IEL. Note that the restraining material1132 may be made of a different material that has a greater resistance(and therefore a slower degradation/stretching rate) todegrading/stretching than the restraining material 1134. Alternatively,the slower degradation/stretching rate of the restraining material 1132may be achieved by applying a thicker coating of the restrainingmaterial 1134 (e.g. twice the thickness, etc.).

It should be understood that while the embodiment of FIG. 11 has beendescribed as having only intermediate and central restrained regions1170 and 1180, the present invention is not limited thereto, and thestent structure 1100 may have a plurality of intermediate restrainedregions having different degradation or stretching characteristics toachieve a smoother radial force gradient. Preferably, thedegradation/stretching rates of the intermediate regions increase fromboth ends of the stent structure 1100 in a direction toward the stentstructure's 1100 longitudinal center. It is also preferable that thedegradation/stretching rate for each region, including the centralregion 1180, increases by a multiple of the outermost intermediateregion 1170. For example, in a stent structure 1100 having twointermediate regions disposed on either side of the longitudinal centerof the stent structure 1100, if the outermost intermediate region (firstintermediate region) is configured to degrade in one week, the secondintermediate regions, which are disposed adjacent to the firstintermediate regions in an inboard direction, are configured to degradein two weeks, while the central region 1180 is configured todegrade/stretch in 3 weeks, etc.

The degradation/stretching rate of the restraining material 140, 240 isbased in large part on the properties of the material itself, as well asthe thickness of the restraining material 140, 240. In some embodiments,the restraining material 140, 240 may comprise multiple layers ofvarying biodegradable polymers, each layer having a differingdegradation rates. Some layers may utilize materials that biodegradequickly while other layers may undergo prolonged degradation. In otherembodiments, the restraining material 140, 240 may comprise a singlelayer of a biodegradable polymer or a composite layer made up of acombination of varying biodegradable polymers. In these embodiments, thesingle layer may degrade at a substantially uniform rate. In otherembodiments, the restraining material 140, 240 may comprise astretchable nonbiodegradable base material that is coated with one ormore layers of biodegradable polymers in varying combinations anddegradation rates. In these embodiments, within a given time periodafter the stent is deployed in a body lumen (preferably within 7 to 180days), the one or more layers of biodegradable polymers will biodegrade,leaving only the nonbiodegradable base material. In other embodiments,the restraining material 140, 240 may comprise a single or multiplelayers of stretchable material, and each layer may be configured tostretch at the same or different rates.

In any of these embodiments, one or more drugs may be incorporated intoone or more layers of the restraining material 140, 240, such asanti-restenosis drugs, anti-inflammatory drugs, anti-thrombotic drugs,and growth factor or growth modulating drugs thereby inhibiting oraccelerating tissue formation. These drugs may be mixed directly intoone or more layers of the restraining material 140, 240 or may beenclosed within a pocket or opening in the restraining material itselffor dispersion after biodegradation begins.

Additionally, the restraining material may be purposely dissolved atdifferent points in time by applying an outside stimulus. For exampleultrasonic heating or oral/systemic administration of an enzyme that istargeted to break down a particular restraining material 140, 240.

While the preceding description of embodiments of the present inventionhave been made with regard to self-expanding stents having specificgeometrical structures, it should be understood that the presentinvention is not limited thereto, and the restraining material 140, 240may be applied to any self-expanding stent having stent cells 130, 230at least partially defined by circumferentially adjacent structuralmembers of any geometry.

The gradually self-expanding stent may be manufactured by compressingthe stent structure 100, 200 and inserting it into a cannula or otherrigid structure. However, it should be understood that the rigidstructure is not limited thereto, and may have any shape capable ofrestraining substantially the entirety of the stent in a compressedconfiguration. The cannula or other rigid structure preferably includesapertures that correspond to stent cells 130, 230 of the stent structure100, 200. The apertures may have any number of shapes, including forexample and without limitation, horizontally extending bars, circles,parallelograms, ovals, and the like. Further, the apertures may beapproximately the same size and shape as the stent cells 130 in thecompressed configuration, or the apertures may be larger than the areaof the stent cells 130, 230. However, it is preferable that theapertures not be smaller than the area of the stent cells 130, 230 sothat substantially the entirety of the area of the stent cell 130, 230corresponding to the aperture can be covered by the restraining material140, 240. Alternatively, the rigid structure may have a substantiallyopen form, and the portions of the stent structure 100, 200 which therestraining material 140, 240 is not to be applied to may be masked offusing adhesive tape, statically charged polymers, or the like.

Once the stent is housed within the rigid structure in its compressedconfiguration, the restraining material is applied to the outer surfaceof the stent structure 100, 200 through the apertures in the cannula.The restraining material may also be applied directly to the outside ofthe stent structure 100, 200 in the case of a masked stent. Therestraining material may be applied by painting, spraying, pouring, orelectro-spinning of particles, fibers, fluids or the like.

In embodiments constructed of super-elastic, shape-memory materials,such as nitinol, the stent structure may be cooled below its austenitetemperature to facilitate compression.

Once the initial layer of restraining material 140, 240 is applied,subsequent layers of restraining material 140, 240 may be applied. Thesesubsequent layers may be of the same or a different material. Further,the stent structure 100, 200 may be removed from the initial rigidstructure and inserted into another rigid structure having aperturescorresponding to different cells than the initial rigid structure, orcorresponding to some of the apertures of the initial rigid structure.In this way, the restraining material 140, 240 covering different stentcells 130, 230 can be made thicker in some cells than others and/or madeof multiple layers of different materials. Alternatively, subsequent andsuccessive layers of material may be applied to select stent cells 130,230 by masking off apertures of the initial rigid structure betweencoatings.

FIGS. 5-7 and 8-10 illustrate the operation of the graduallyself-expanding stent shown in FIG. 4. FIG. 4 illustrates a graduallyself-expanding stent structure 400 in a compressed form prior todeployment in a body lumen. The stent structure 400 includes of aplurality of interconnected structural members 410 that define aplurality of stent cells 430. Each stent cell 430 has an area that isdefined by the circumferentially adjacent structural members 420. Arestraining material 440 is attached to the circumferentially adjacentstructural members 420 of a plurality of stent cells 430 such that therestraining material 440 substantially covers the entirety of the areaof the stent cell, thereby connecting each of the structural members 420and restraining the stent cell 430 in a compressed configuration. Therestrained stent cells 430 are disposed in a pattern on the stentstructure 400 such that the restraining material 440 does not extendaround the circumference of the stent structure 400 so as to form acontinuous circumferential band of restraining material 440.Additionally, the majority of the stent cells 430 to which therestraining material 440 is attached are located in a longitudinallycentral portion of the stent structure 400. The restraining material 440is not attached to any of the stent cells 430 disposed at either end ofthe stent structure 440.

Initially, the stent structure 400 is inserted in a low-profilecompressed configuration into the end of a delivery catheter. Typically,the delivery catheter includes a restraining sheath, and an innercatheter that includes a stop extending radially outward from a guidewire lumen of the inner catheter. A distal surface of the stop isdisposed adjacent a proximal end of the stent structure 400.

In operation, initially, a guide-wire is advanced through a trocar intoa desired vessel or cavity using the Seldinger technique, which isconventional and well known in the art. The guide-wire is then advancedthrough the patient's vasculature or cavity until it reaches the desiredtreatment site, for example a lesion/stricture 20 on a vessel wall 10.Once the guide-wire is in the desired position, the delivery catheter isthen inserted into a patient's vasculature over the guide-wire, andadvanced to the lesion/stricture 20 by sliding the delivery system alongthe guide-wire in a distal direction.

The stent structure 400 may be positioned at the lesion/stricture 20using radiopaque markers located on the stent structure 400. Theradiopaque markers allow a physician to visualize the stent structure400 from outside the patient's body using x-ray fluoroscopy.

Once the stent is in position at the lesion/stricture 20, the physicianretracts the retention sheath in the proximal direction using a controlhandle or the like. As the retention sheath is drawn in the proximaldirection relative to the inner catheter, the stent structure 400 ispushed out of the retention sheath by the stop.

FIGS. 5-7 illustrate the operation of an embodiment of the stentstructure 400 utilizing rigid, biodegradable restraining material 440.As shown in FIG. 5, once the stent structure 400 is released from theretention sheath, the proximal and distal ends of the stent structure400, which do not have stent cells 430 restrained by restrainingmaterial 440, immediately expand in a radially outward direction andappose the wall of the blood vessel 10. Preferably, the stent structureis positioned such that the proximal and distal ends of the stentstructure 400 contact healthy tissue when expanded. In addition to thestent cells 430 disposed at the proximal and distal ends of the stentstructure 400, upon deployment, each of the stent cells 430 that is notcovered by, or attached to the restraining material 440 immediatelyexpands to the maximum degree allowable. In contrast, each of the stentcells 430 that is covered by, and attached to the restraining material440 generally maintains the same compressed, low-profile configurationas it had prior to deployment. Thus, the portions of the stent structure400 containing restrained stent cells 430 are unable to fully expand,and assume a partially expanded configuration. The degree to which eachportion of the stent structure 400 expands is directly related to thenumber of stent cells 430 that are covered by restraining material 440.Thus, the more stent cells 430 that are covered by restraining material440 in a given area of the stent structure 400, the more compressed thatarea of the stent structure 400 remains. Preferably, the restrainedstent cells correspond to the location and geometry of thelesion/stricture 20, thus preventing the stent structure 400 fromexerting radial force against the lesion/stricture 20 and minimizing thepotential for rupturing or damaging the IEL. Because the majority of therestrained stent cells are disposed in a longitudinally central portionof the stent structure 400, the stent structure 400 initially assumes anhourglass-like shape with the ends of the stent structure apposinghealthy tissues of the vessel wall 10 and the center of the stentstructure contacts, but does not apply significant outward force againstthe inner surface of the stricture/lesion 20. Preferably, the stentstructure 100, 200 is not restrained away from the narrowest portion ofthe stricture/lesion 20 to avoid gaps that may promote the formation ofthrombi.

As shown in FIGS. 6-7, over time, and preferably between seven (7) andone-hundred-eighty (180) days the restraining material 440 begins todegrade and release the restrained stent cells 430. Once the restrainingmaterial 440 of a sufficient number of stent cells 430 has degraded, thestent structure 440 begins to exert force in a radially outwarddirection against the lesion/stricture 20, thereby slowly and graduallyforcing the lesion/stricture 20 to expand outward and open the bloodvessel. Eventually, after all of the restraining material 440 isdissolved and all of the stent cells 430 assume their expandedconfiguration, the stent structure 400 opens the lesion/stricture 20 to,or close to the diameter of the vessel adjacent to the lesion/stricture20.

FIGS. 8-10 illustrate the operation of an embodiment of the stentstructure 500 utilizing a stretchable restraining material 540. Thestretchable restraining material 540 may be biodegradable,nonbiodegradable, or a combination thereof. As with the stent structure400 utilizing rigid restraining material 440 described above, once thestent structure 500 is released from the retention sheath, the proximaland distal ends of the stent structure 500, which do not have stentcells 530 restrained by restraining material 540, immediately expand ina radially outward direction and appose the wall of the blood vessel 10.Preferably, the stent structure is positioned such that the proximal anddistal ends of the stent structure 500 contact healthy tissue whenexpanded. In addition to the stent cells 530 disposed at the proximaland distal ends of the stent structure 500, upon deployment, each of thestent cells 530 that is not covered by, or attached to, the restrainingmaterial 540 immediately expands to the maximum degree allowable. Incontrast, each of the stent cells 530 that is covered by, and attachedto the restraining material 540, maintains the same compressed,low-profile configuration, or a slightly larger fixed initial profile asit had prior to deployment. Thus, the portions of the stent structure500 containing restrained stent cells 530 are initially unable to expandagainst the vessel wall 10, and assume a partially expandedconfiguration. Preferably, the restrained stent cells correspond to thelocation and geometry of the lesion/stricture 20, thus preventing thestent structure 500 from exerting radial force against thelesion/stricture 20 and minimizing the potential for rupturing ordamaging the IEL. Because the majority of the restrained stent cells aredisposed in a longitudinally central portion of the stent structure 500,the stent structure 500 initially assumes an hourglass-like shape withthe ends of the stent structure apposing healthy tissues of the vesselwall 10, and the center of the stent structure contacts, but does notapply significant outward force against the inner surface of thestricture/lesion 20.

As shown in FIGS. 8-10, over time, and preferably between seven (7) andone-hundred-eighty (180) days the restraining material 540 begins tostretch, thereby increasing the area of the restrained stent cells 530and slowly allowing the stent structure 500 to expand. As therestraining material 540 continues to stretch, the stent structure 540eventually begins to exert force against the lesion/stricture 20 in aradially outward direction. This outward pressure against thelesion/stricture slowly and gradually forces it to expand outward,thereby opening the blood vessel. Eventually, after the restrainingmaterial 540 is completely stretched out, the stent cells 530 assumetheir expanded configuration, and the stent structure 500 opens thelesion/stricture 20 to, or close to the diameter of the vessel adjacentto the lesion/stricture 20.

Accordingly, gradually self-expanding stents 400, 500 employing eitherrigid or stretchable restraining material, or a combination thereof,slowly apply radial force against the lesion/stricture 20, therebyallowing the IEL sufficient time to remodel and stretch without ruptureor damage.

FIG. 12 illustrates an alternative embodiment of the graduallyself-expanding stent 100, 200 that is designed to be used in conjunctionwith one or more gradually self-expanding stent structures 100, 200, ora conventional self-expanding stent, in an overlapping manner to treatvery long lesions/strictures 20 (e.g. lesions/strictures exceeding alength of 100 mm), as shown in FIG. 13. The self-expanding stent 1200may consist of two portions, a distal portion 1220, and a proximalportion 1230. The distal portion extends from the distal end 1250 towardthe proximal end 1240 of the stent structure 1200 by a distance (a),which may be, for example, between 5 and 10 millimeters. The proximalportion 1230 extends from the proximal end of the distal portion 1222 tothe proximal end of the stent structure 1240. The proximal portion 1230includes a plurality of stent cells 1270. A restraining material 1260 isapplied to the stent struts 1280 defining some of the stent cells 1270(preferably 50%) in the proximal portion 1230 such that the restrainingmaterial 1260 covers the substantially the entire area of the stentcells 1270. In a preferred embodiment, the proximal portion 1230 mayinclude some restrained stent cells 1234 disposed at a longitudinallydistal portion covered in a restraining material 1264 that is configuredto degrade or stretch more slowly than the restrained stent cells 1232covered in a restraining material 1260, as described above inconjunction with FIG. 3. However, unlike the embodiment shown in FIG. 3,the restrained stent cells 1232 extend all the way to the proximal end1240 of the stent structure 1200. Thus, when the stent structure 1200 isdeployed in a body lumen, only the distal portion 1220 is free toimmediately expand out and appose the inner wall of the body lumen,while the proximal end 1240 remains in the same partially expandedconfiguration as the proximal portion 1230.

As shown in FIG. 13, when the stent structure 1200 (the “primary stent”)is deployed in a body lumen 10 having a long lesion/stricture 20, thedistal portion of the stent structure 1200 expands out to, and apposesthe inner diameter of the body lumen 10 such that the distal end 1250 ofthe stent structure 1200 is anchored to the healthy tissue of the bodylumen 10 and will not migrate within the body lumen 10. Once the stentstructure 1200 is deployed, a second self-expanding stent having aconfiguration similar to the stent structure 100, 200 (the secondarystent) described above with regard to FIGS. 1-3, is guided to thetreatment site using the Seldinger technique such that the distal end110, 210 of the stent structure 100, 200 is inserted into the proximalend 1240 of the stent structure 1200. The secondary stent 100, 200 isthen advanced into the proximal end 1240 of the primary stent 1200 suchthat the distal portion of the secondary stent 100, 200, which consistsof open and unrestrained stent cells 1236, is disposed inside theprimary stent 1200. The secondary stent 100, 200 is then released andthe unrestrained distal portion of the secondary stent 100, 200immediately expands out to the inner diameter of the primary stent 1200,such that the outer surface of the stent struts in the distal portion ofthe secondary stent 100, 200 appose the inner surface of correspondingstent struts of the proximal portion of the primary stent 1200. Oncedeployed, the combination of the primary stent 1200 and the secondarystent 100, 200 results in a gradually self-expanding stent system havingproximal and distal ends that appose healthy tissue of the body lumen 10at either end of the lesion/stricture 20, and that remains partiallycompressed in the central portion corresponding to the lesion/stricture20.

Note that the increased outward radial force exerted on the portion ofthe primary stent 1200 by the distal portion of the secondary stent 100,200 preferably does not cause that portion of the primary stent 1200 toinitially expand when the secondary stent 100, 200 is deployed. In analternative embodiment, the restraining material 1260 covering the stentcells 1270 of the proximal portion 1230 that is likely to interface withthe distal portion of the secondary stent 100, 200 may be modified tocounteract the additional outward radial force exerted by the secondarystent 100, 200. For example, the restraining material 1260 in theseareas may be thicker, or made of a stronger material.

While preferred embodiments of the invention have been described, itshould be understood that the invention is not so limited, andmodifications may be made without departing from the invention. Thescope of the invention is defined by the appended claims, and alldevices that come within the meaning of the claims, either literally orby equivalence, are intended to be embraced therein. Furthermore, theadvantages described above are not necessarily the only advantages ofthe invention, and it is not necessarily expected that all of thedescribed advantages will be achieved with every embodiment of theinvention.

1. A self-expanding stent, comprising: a stent structure comprising aplurality of stent cells having a compressed configuration and anexpanded configuration, said stent structure being self-expanding fromsaid compressed configuration to said expanded configuration; a firstregion and two second regions of said stent structure, said first regionbeing disposed longitudinally between said second regions; a firstrestraining material attached to said first region of said stentstructure, said first restraining material initially restraining saidfirst region from expanding to said expanded configuration, and saidfirst restraining material releasing said first region over time toallow said first region to expand to said expanded configuration; andwherein said second regions of said stent structure expand at a fasterrate than said first region.
 2. The self-expanding stent according toclaim 1, wherein said second regions are open and unrestrained by arestraining material, said second regions thereby being free toimmediately expand to said expanded configuration.
 3. The self-expandingstent according to claim 1, wherein said second regions are disposed ata most proximal end and a most distal end of said stent structure. 4.The self-expanding stent according to claim 1, further comprising twothird regions, said first region and said second regions being disposedlongitudinally between said third regions, and a second restrainingmaterial attached to said second regions of said stent structure, saidsecond restraining material initially restraining said second regionsfrom expanding to said expanded configuration, and said secondrestraining material releasing said second regions over time to allowsaid second regions to expand to said expanded configuration, whereinsaid second restraining material releases said second regions at afaster rate than said first restraining material releases said firstregion.
 5. The self-expanding stent according to claim 4, wherein saidthird regions are open and unrestrained by a restraining material, saidthird regions thereby being free to immediately expand to said expandedconfiguration.
 6. The self-expanding stent according to claim 5, whereinsaid third regions are disposed at a most proximal end and a most distalend of said stent structure.
 7. The self-expanding stent according toclaim 1, wherein said first region of said stent structure comprises afirst plurality of said stent cells, each of said first plurality ofsaid stent cells in said first region comprising first and second bends,said first and second bends facing each other and being longitudinallyspaced apart, a plurality of structural members interconnecting saidfirst and second bends, wherein each structural member has a length,each of said first plurality of said stent cells being defined by saidfirst and second bends and said lengths of said plurality of structuralmembers, and said first restraining material is attached to said firstplurality of said stent cells along the entire length of said structuralmembers, said first restraining material extending across an entirety ofan area defined by said first and second bends and said plurality ofstructural members of each of said stent cells in said first pluralityof said stent cells, said first restraining material thereby extendinglongitudinally across each of said areas between said first and secondbends and circumferentially across each of said areas between saidplurality of structural members disposed adjacent each other.
 8. Theself-expanding stent according to claim 7, wherein said first region ofsaid stent structure further comprises a second plurality of said stentcells dispersed amongst said first plurality of said stent cells in acircumferentially uniform manner, said second plurality of said stentcells being open and unrestrained by a restraining material, said secondplurality of said stent cells thereby being free to immediately expand.9. The self-expanding stent according to claim 8, wherein a proportionof said first plurality of said stent cells to said second plurality ofsaid stent cells is substantially equal.
 10. The self-expanding stentaccording to claim 1, wherein said first region is disposed at alongitudinally central portion of said stent structure.
 11. Theself-expanding stent according to claim 10, wherein said first region isbetween 40% to 80% of a total length of said stent structure.
 12. Theself-expanding stent according to claim 1, wherein said firstrestraining material is biodegradable.
 13. The self-expanding stentaccording to claim 1, wherein said first region is disposed at alongitudinally central portion of said stent structure, said firstrestraining material is biodegradable, said second regions are open andunrestrained by a restraining material, said second regions therebybeing free to immediately expand to said expanded configuration, andsaid second regions are disposed at a most proximal end and a mostdistal end of said stent structure.
 14. The self-expanding stentaccording to claim 13, wherein said first region of said stent structurecomprises a first plurality of said stent cells, each of said firstplurality of said stent cells in said first region comprising first andsecond bends, said first and second bends facing each other and beinglongitudinally spaced apart, a plurality of structural membersinterconnecting said first and second bends, wherein each structuralmember has a length, each of said first plurality of said stent cellsbeing defined by said first and second bends and said lengths of saidplurality of structural members, and said first restraining material isattached to said first plurality of said stent cells along the entirelength of said structural members, said first restraining materialextending across an entirety of an area defined by said first and secondbends and said plurality of structural members of each of said stentcells in said first plurality of said stent cells, said firstrestraining material thereby extending longitudinally across each ofsaid areas between said first and second bends and circumferentiallyacross each of said areas between said plurality of structural membersdisposed adjacent each other.
 15. The self-expanding stent according toclaim 14, wherein said first region of said stent structure furthercomprises a second plurality of said stent cells dispersed amongst saidfirst plurality of said stent cells in a circumferentially uniformmanner, said second plurality of said stent cells being open andunrestrained by a restraining material, said second plurality of saidstent cells thereby being free to immediately expand.
 16. Theself-expanding stent according to claim 15, wherein said first region isbetween 40% to 80% of a total length of said stent structure.
 17. Theself-expanding stent according to claim 1, further comprising two thirdregions, said first region and said second regions being disposedlongitudinally between said third regions, and a second restrainingmaterial attached to said second regions of said stent structure, saidsecond restraining material initially restraining said second regionsfrom expanding to said expanded configuration, and said secondrestraining material releasing said second regions over time to allowsaid second regions to expand to said expanded configuration, whereinsaid second restraining material releases said second regions at afaster rate than said first restraining material releases said firstregion, said first restraining material is biodegradable, said firstregion is disposed at a longitudinally central portion of said stentstructure, and said third regions are disposed at a most proximal endand a most distal end of said stent structure.
 18. The self-expandingstent according to claim 17, wherein said third regions are open andunrestrained by a restraining material, said third regions thereby beingfree to immediately expand to said expanded configuration.
 19. Theself-expanding stent according to claim 18, wherein said first region ofsaid stent structure comprises a first plurality of said stent cells,each of said first plurality of said stent cells in said first regioncomprising first and second bends, said first and second bends facingeach other and being longitudinally spaced apart, a plurality ofstructural members interconnecting said first and second bends, whereineach structural member has a length, each of said first plurality ofsaid stent cells being defined by said first and second bends and saidlengths of said plurality of structural members, and said firstrestraining material is attached to said first plurality of said stentcells along the entire length of said structural members, said firstrestraining material extending across an entirety of an area defined bysaid first and second bends and said plurality of structural members ofeach of said stent cells in said first plurality of said stent cells,said first restraining material thereby extending longitudinally acrosseach of said areas between said first and second bends andcircumferentially across each of said areas between said plurality ofstructural members disposed adjacent each other, and said first regionof said stent structure further comprises a second plurality of saidstent cells dispersed amongst said first plurality of said stent cellsin a circumferentially uniform manner, said second plurality of saidstent cells being open and unrestrained by a restraining material, saidsecond plurality of said stent cells thereby being free to immediatelyexpand, and said second regions of said stent structure comprises athird plurality of said stent cells, each of said third plurality ofsaid stent cells in said second regions comprising first and secondbends, said first and second bends facing each other and beinglongitudinally spaced apart, a plurality of structural membersinterconnecting said first and second bends, wherein each structuralmember has a length, each of said third plurality of said stent cellsbeing defined by said first and second bends and said lengths of saidplurality of structural members, and said second restraining material isattached to said third plurality of said stent cells along the entirelength of said structural members, said second restraining materialextending across an entirety of an area defined by said first and secondbends and said plurality of structural members of each of said stentcells in said third plurality of said stent cells, said secondrestraining material thereby extending longitudinally across each ofsaid areas between said first and second bends and circumferentiallyacross each of said areas between said plurality of structural membersdisposed adjacent each other, and said second regions of said stentstructure further comprises a fourth plurality of said stent cellsdispersed amongst said third plurality of said stent cells in acircumferentially uniform manner, said fourth plurality of said stentcells being open and unrestrained by a restraining material, said fourthplurality of said stent cells thereby being free to immediately expand.20. The self-expanding stent according to claim 19, wherein a proportionof said first plurality of said stent cells to said second plurality ofsaid stent cells is substantially equal, a proportion of said thirdplurality of said stent cells to said fourth plurality of said stentcells is substantially equal, and said first region is between 40% to80% of a total length of said stent structure.